1. Field of the Invention
The present invention relates to a mechanism for driving photosensitive drums in a color recording apparatus.
2. Description of the Prior Art
One known type of color recording apparatus is a color copying machine. The color copying machine comprises a plurality of photosensitive drums, charging devices associated respectively with the photosensitive drums, optical systems including convergent light transmitting arrays and other members for illuminating a colored original document and exposing the photosensitive drums to unmagnified light images of the original document, image developing devices, and image transfer devices. Color-separated light images are formed respectively on the photosensitive drums and then developed into respective visible images thereon, which are thereafter transferred to a single image transfer sheet in overlapping relation (see Japanese Laid-Open Patent Publication No. 57-85066, for example).
In the above conventional copying copying machine, the colors would tend to be imposed out of register resulting in a fuzzy image if the images on the respective photosensitive drums were not exactly positioned with respect to the image transfer sheet. The reasons for such a problem is that the mechanical components of the color copying machine are manufactured with high accuracy. However, when these components are assembled and operated, they are liable to vary in their motions. For example, when the photosensitive drums make one revolution, they may rotate at the same speed on average, but variations in the speed may differ from drum to drum during the revolution of the photosensitive drums. Therefore, at the time the images are transferred successively from the photosensitive drums onto one image transfer sheet, the transferred images are apt to be out of registry since the peripheral speeds of the photosensitive drums reach their maximum and minimum levels at different times.
There has been proposed a mechanism for driving the photosensitive drums in order to prevent the images from being positioned out of registry on the image transfer sheet. This mechanism includes a plurality of gears on the respective shafts of the photosensitive drums, a train of gears positioned between and held in mesh with the gears on the drum shafts, and a single driving means such as an electric motor coupled directly or indirectly to one of the gears of the gear train. The gears of the gear train are of identical shape, molded by one mold or simultaneously integrally. When these gears are assembled, the same angular positions on the gears are required to be kept in certain phase with respect to each other according to the speed variations of the shafts of the photosensitive drums (see, for example, Japanese Laid-Open Patent Publication Nos. 61-156158 and 61-156159).
However, the prior mechanism for driving the photosensitive drums is problematic as follows: The backlash produced by each of the gears of the gear train affects the intermediate gears up to the final gear inasmuch as the gears are driven successively in series by the single motor. As a consequence, even if the relative angular positions of the gears on the drum shafts are adjusted such that these gears will vary in the speed in the same pattern, the gears start rotating at different times due to the accumulated gear backlash, with the result that the image positions on the respective photosensitive drums will reach the image transfer position at different times. In the event, the photosensitive drums are subjected to rotation irregularities as described above, the images on the photosensitive drums are caused to be out of registry with the image on the image transfer sheet.